Processes for producing spray-dried pregelatinized starch have been disclosed in U.S. Pat. No. 4,280,851 to Pitchon et al. This patent, which is hereby incorporated by reference, discloses a process wherein an aqueous starch slurry is injected through one or more centrally-located atomization apertures in a two-fluid nozzle to form a relatively fine-sized spray. A heating medium (e.g. steam) is also injected through other apertures in the nozzle and into the spray of atomized starch slurry so as to heat the starch to a temperature which is effective to gelatinize the starch. An enclosed chamber surrounds the atomization and heating medium injection apertures and defines a vent aperture which is positioned to enable the heated, atomized starch spray to exit the chamber. The chamber is sized such that the elapsed time for passage of the spray droplets through the chamber (i.e., from the atomization aperture through the vent aperture) is sufficient to effect gelatinization of the starch. The major portion of gelatinization occurs within the chamber; however, some gelatinization may occur upon the exiting of the spray from the enclosed chamber, as a result of the heat and moisture that is carried from the chamber. Thus, gelatinization time includes both time within the chamber as well as the period of time outside of the chamber during which the starch retains a temperature and moisture which can effect gelatinization.
The Pitchon et al. patent further discloses that the spray from the enclosed chamber passes into a spray-drying tower where the spray particles are dried in a stream of heated air. This patent also discloses that a plurality of nozzles may be uniformly-spaced around the top of a single spray-drying tower in order to increase the amount of spray-dried, pregelatinized starch material which is produced. The dried, pregelatinized starch produced in accordance with the Pitchon et al. patent is described as having a mesh size wherein about 80% by weight of the starch passes through a 230 mesh U.S. Standard Screen, with the particles being in the form of loosely-bound agglomerates of starch granules, the granules being in the form of indented spheres and the agglomerates separating into individual granules upon dispersion in water.
The spray-dried starch material produced by the process of the Pitchon et al. patent has been found to contain both an exceedingly wide range of particle sizes and a high percentage of fine particles (e.g., 55% by weight of the particles being less than 53 microns or minus 270 U.S. mesh). It is desirable to produce a spray-dried pregelatinized starch material which has a relatively narrow particle size distribution. Such a material could more readily be optimized to meet the preferred particle size requirements of a given starch-containing, dry mix. It is also desirable to obtain a spray-dried pregelatinized starch material which has a reduced level of fine (minus 270 U.S. mesh) particles. This will improve the dispersibility of the starch when a dry-blended mix containing the starch is mixed with a cold, aqueous fluid, such as when instant pudding mixes ar combined with milk.
Of course, it is known to adjust particle size distribution and reduce the level of fines in a particulate material by means of agglomeration. Fine starch powders are typically agglomerated using balling or tumbling techniques wherein a liquid binder is sprayed onto an agitated bed of dry starch powder. The liquid dissolves the surface of the starch particles and liquid bridges are formed between solid particles which bridges, upon drying, become a solid bond. Such agglomeration techniques, however, require separate agglomeration equipment and processing and thus add to production costs.